The nacelle of a wind turbine usually comprises a plurality of bearings, for a rotor shaft, gearbox, electrical generator, yaw gearbox, yaw slewing table, blade pitch revolving seat and/or hydraulic pump for example. The bearing loads and rotating speeds to which such bearings are subjected during their use vary considerably due to changing wind speeds and directions. At wind speeds below the cut-in wind speed (i.e. the minimum wind speed required for power generation) a rotor shaft will idle, resulting in low speed, low load operation. At wind speeds above the cut-in speed the rotor shaft will rotate, resulting in high speed, high load operation. The bearings of a wind turbine may therefore be subjected to repeated start, acceleration, deceleration and stop operations. Furthermore, the bearing may be exposed to the elements and airborne particles, such as sand particles, and will therefore need to be of high strength and hardness.
Carbonitriding is a metallurgical surface modification technique that is used to increase the surface hardness of metal components, thereby reducing the wear of the components during use. During the carbonitriding process, atoms of carbon and nitrogen diffuse interstitially into the metal, creating barriers to slip and increasing the hardness near the surface, typically in a layer that is 0.1 to 0.3 mm thick. Carbonitriding is usually carried out a temperature of 850-860° C. Carbonitriding is normally used to improve the wear resistance of steel components comprising low or medium carbon steel, and not high carbon steel. Although steel components comprising high carbon steel are stronger, they have been found to be more susceptible to cracking in certain applications.